On conventional spinning spacecraft, such as for the Giotto and Cluster missions, the ground is entirely in charge of attitude determination and the design of momentum re-orientation manoeuvres: the ground performs the attitude reconstitution through measurements from a (non-autonomous) star mapper and computes the next manoeuvre to be executed in near real time. The manoeuvre is computed and executed “a priori”, from the knowledge of the actual spin axis attitude, and the prescribed spin axis attitude. This is illustrated in FIG. 1.
This known approach reduces/minimises the on-board AOCMS software complexity, but at the expense of ground computations, and during non-contact periods, manoeuvre realisation errors accumulate. It also requires a specific solution to master the nutation generated by the momentum reorientation manoeuvres. In this connection, the following requirements are to be noted:    (1) No closed loop control is required on-board, allowing for a simpler AOCMS on-board; closing the loop is performed on-ground. No on-board autonomous attitude determination is therefore required, and no on-board closed loop computation and execution of manoeuvres is therefore required.    (2) Nutation Avoidance Manoeuvres strategies for momentum re-orientation manoeuvres are required to maximise the useful mission time (i.e. when the pointing stabilisation requirement is met).
In this known strategy, as illustrated in FIG. 2, the manoeuvre is split into two small pulses. Nutation is shown to be minimised by phasing the two pulses with respect to each other. The thrust angle is a function of the spin rate (to perform a thrust in the right direction with respect to the inertial frame), and of the nutation rate (to perform a thrust that “kills” the nutation of the first thrust in the body frame). The efficiency of the Nutation Avoidance Manoeuvre is directly given by the thrust angle. Some limitations are however inherent to this known strategy. For example, for some spacecraft inertial tensor properties, it is not possible to find both a thrust angle and a thrust phasing that null the nutation at the end of the spin axis re-orientation. This kind of open loop Nutation Avoidance Manoeuvre is inefficient for some unfavourable inertia ratios, as shown in FIG. 3, thus, and requires the inertial nutation period to be phased with the body nutation period. In case no phasing is available (within a short time, typically before two spin periods), there is an unwanted residual nutation at the end of the second manoeuvre.    (3) Residual nutation control is performed passively, using Passive Nutation Dampers. Note that a Passive Nutation Damper (PND) typically comprises a tube and end pots filled with fluid. Nutation creates cyclic acceleration along the tube and energy dissipation within the fluid makes the nutation decay, so motion tends to a pure spin around the principal inertia axis. PNDs are tuned on the nutation frequency. The tuning is achieved by appropriately designing their dimensions with respect to the dynamic characteristics of the spacecraft. The resulting PND time constant depends on the spacecraft geometry, on the spin rate and on the PND intrinsic characteristics.
In the framework of the new generation of mission, however, the increased autonomy requirements (aimed at reducing the operational costs) and the more complex spin momentum attitude requirements (aimed at providing sufficient manoeuvrability, as required by the mission sky or Earth coverage) do not make near real time operations using the above described known approach simple. To fulfil the requirements of the new generation missions, attitude determination has to be done on a typical 24 hours basis, and manoeuvres have to be computed and executed, “a priori”, from the initial attitude determination solution for, typically, the coming 48 hours.
Further, error accumulation increases as momentum re-orientation manoeuvre realisation errors accumulate. As an example, when using thrusters, these are mainly linked to thruster impulse bit repeatability and sensitivity to initial actual thruster temperature. Error accumulation yields a pointing drift error of typically a few arc minutes per day, depending on the actual correlation between independent manoeuvres. Usage of a solution, as currently defined for former spinning satellites, can be detrimental to the required medium term pointing stability for the mission.
From the short term pointing stability point of view, it is desired to minimise the nutation generated by the momentum re-orientation manoeuvres, and also to minimise the subsequently required time to damp any residual nutation. Both items are needed in order to reach a “steady-state” pointing stability performance as soon as possible. This in turn maximises useful mission time. Tightly controlling the nutation principally through Nutation Avoidance Manoeuvres using known arrangements, however, can either cause constraints on the spacecraft mass properties, or require a strategy redesign if the mass properties evolve.
On the other hand, the usage of known Passive Nutation Dampers cause accommodation and validation issues for spinning spacecraft. This is particularly true when a short duration is available after spacecraft momentum repainting manoeuvres.